Cellular signal transduction is a fundamental mechanism whereby external stimuli that regulate diverse cellular processes are relayed to the interior of cells. These processes include, but are not limited to, cell proliferation, differentiation and survival. Many tyrosine kinases are expressed in postmitotic, fully differentiated cells, particularly in the case of hematopoietic cells, and it seems likely that these proteins are involved in specialized cellular functions that are specific for the cell types in which they are expressed. (Eiseman, E. and J. B. Bolen, Cancer Cells 2(10):303-310, 1990). A central feature of signal transduction is the reversible phosphorylation of certain proteins. (for reviews, see Posada, J. and Cooper, J. A., 1992, Mol. Biol. Cell 3:583-392; Hardie, D. G., 1990, Symp. Soc. Exp. Biol. 44:241-255). The phosphorylation state of a protein is modified through the reciprocal actions of tyrosine kinases (TKs), which function to phosphorylate proteins, and tyrosine phosphatases (TPs), which function to dephosphorylate proteins. Normal cellular function requires a delicate balance between the activities of these two types of enzyme.
Phosphorylation of cell surface tyrosine kinases, stimulates a physical association of the activated receptor with intracellular target molecules. Some of the target molecules are in turn phosphorylated. Other target molecules are not phosphorylated, but assist in signal transmission by acting as adapter molecules for secondary signal transducer proteins.
The secondary signal transducer molecules generated by activated receptors result in a signal cascade that regulates cell functions such as cell division or differentiation. Reviews describing intracellular signal transduction include Aaronson, S. A., Science 254:1146-1153, 1991; Schlessinger, J. Trends Biochem. Sci. 13:443-447, 1988; and Ullrich, A., and Schlessinger, J. Cell 61:203-212, 1990.
Receptor tyrosine kinases are composed of at least three domains: an extracellular ligand binding domain, a transmembrane domain and a cytoplasmic catalytic domain that can phosphorylate tyrosine residues. The intracellular, cytoplasmic, non-receptor protein tyrosine kinases may be broadly defined as those protein tyrosine kinases which do not contain a hydrophobic, transmembrane domain. Bolen (Oncogene, vol. 8, pgs. 2025-2031 (1993)) reports that 24 individual protein tyrosine kinases comprising eight different families of non-receptor protein tyrosine kinases have been identified: Abl/Arg; Jak1/Jak2/Tyk2; Fak; Fes/Fps; Syk/Zap; Tsk/Tec/Atk; Csk; and the Src group, which includes the family members Src, Yes, Fyn, Lyn, Lck, Blk, Hck, Fgr and Yrk. All of the non-receptor protein tyrosine kinases are thought to be involved in signaling pathways that modulate growth and differentiation. Bolen, supra, suggests that half of the nonreceptor protein tyrosine kinases have demonstrated oncogenic potential and half appear to be primarily related to suppressing the activity of Src-related protein kinases and could be classified as anti-oncogenes.
While distinct in their overall molecular structure, each member of a given morphotypic family of cytoplasmic protein tyrosine kinases shares sequence homology in certain non-catalytic domains in addition to sharing sequence homology in the catalytic kinase domain. Examples of defined non-catalytic domains include the SH2 (SRC homology domain 2; Sadowski, I et al., Mol. Cell. Biol. 6:4396-4408; Kock, C. A. et al., 1991, Science 252:668-674) domains, SH3 domains (Mayer, B. J. et al., 1988, Nature 332:269-272) and PH domains (Musacchio et al., TIBS 18:343-348 (1993). These non-catalytic domains are thought to be important in the regulation of protein-protein interactions during signal transduction (Pawson, T. and Gish, G., 1992, Cell 71:359-362).
While the metabolic roles of cytoplasmic protein tyrosine kinases are less well understood than that of the receptor-type protein tyrosine kinases, significant progress has been made in elucidating some or the processes in which this class of molecules is involved. For example, members of the src family, lck and fyn, have been shown to interact with CD4/CD8 and the T cell receptor complex, and are thus implicated in T cell activation, (Veillette, A. Davidson, D., 1992, TIG 8:61-66). Some cytoplasmic protein tyrosine kinases have been linked to certain phases of the cell cycle (Morgan, D. O. et al., 1989, Cell 57:775-786; Kipreos, E. T. et al., 1990, Science 248:217-220; Weaver et al., 1991, Mol. Cell. Biol. 11:4415-4422), and cytoplasmic protein tyrosine kinases have been implicated in neuronal and hematopoietic development (Maness, P., 1992, Dev. Neurosci 14:257-270 and Rawlings et al., Science 261:358-361 (1993)). Deregulation of kinase activity through mutation or overexpression is a well-established mechanism underlying cell transformation (Hunter et al., 1985, supra; Ullrich et al., supra).
A variety of cytoplasmic tyrosine kinases are expressed in, and may have important functions in, hematopoietic cells including src, lyn, fyn, blk, lck, csk and hck. (Eisenian, E. and J. B. Bolen, Cancer Cells 2(10):303-310, 1990). T-cell activation, for example, is associated with activation of lck. The signaling activity of lyn may be stimulated by binding of allergens to IgE on the surface of basophils. (Eisenian, supra).
Abnormalities in tyrosine kinase regulated signal transduction pathways can result in a number of disease states. For example, mutations in the cytoplasmic tyrosine kinase atk (also called btk) are responsible for the x-linked agammaglobulinemia, (Ventrie, D., et al., Nature 361:226-23, 1993). This defect appears to prevent the normal differentiation of pre-B cells to mature circulating B cells and results in a complete lack of serum immunoglobulins of all isotypes. The cytoplasmic tyrosine kinase Zap-70 has been suggested as indispensable for the development of CD8 single-positive T cells as well as for signal transduction and function of single-positive CD4 T cells, and lack of this protein leads of an immunodeficiency disease in humans, (Arpala, E., et al., Cell 76:1-20, 1994). Gene knockout experiments in mice suggest a role for src in the regulation of osteoclast function and bone remodeling as these mice develop osteopetrosis. (Soriano et al., Cell 64:693-702, 1991 and Lowe et al., PNAS (in press)).
Megakaryocytes are large cells normally present in bone marrow and spleen and are the progenitor cell for blood platelets. Megakaryocytes are associated with such disease states as acute megakaryocytic leukemia (Lu et al., Cancer Genet Cytogenet, 67(2):81-89 (1993) and Moody et al., Pediatr Radiol. 19(6-7):486-488 (1989)), a disease that is difficult to diagnose early and which is characterized by aberrant proliferation of immature cells or “blasts”; myelofibrosis (Smith et al., Crit Rev Oncol Hematol. 10(4):305-314 (1990) and Marino, J. Am. Osteopath Assoc. 10:1323-1326 (1989)), an often fatal disease where the malignant cell may be of megakaryocytic lineage and may be mediated by platelet or megakaryocyte growth factors; acute megakaryocytic myelosis (Fohlmeister et al., Haematologia 19(2):151-160 (1986)) a rapidly fatal disease characterized by megakaryocytic proliferation and the appearance of immature megakaryocytes in the circulation; and acute myelosclerosis (Butler et al., Cancer 49(12):2497-2499 (1982) and Bearman et al., Cancer 43(1):279-93 (1979)) a myeloproliferative syndrome where the marrow is characterized by atypical megakaryocytes.
Platelets play a key role in the regulation of blood clotting and wound healing, as well as being associated with such disease conditions as thrombocytopenia, atherosclerosis, restenosis and leukemia. Several receptor tyrosine kinases have been identified in human megakaryocytes including c-kit, blg and blk. (Hoffman, H., Blood 74:1196-1212, 1989; Long, M. W., Stem Cells 11:33-40, 1993; Zaebo, K. M., et al., Cell 63:213-224,1990). Cytoplasmic tyrosine kinases of human megakaryocytic origin have also been reported. (Bennett et al., Journal of Biological Chemistry 289(2):1068-1074, 1994; Lee et al., Gene 1-5, 1993; and Sakano et al., Oncogene 9:1155-1161 (1994)).